Ok I got this game a few months ago and just started playing it and I have to admit it is a very good game. I mean where else can you play with rockets and shoot little green men either into orbit or to their doom?
And the amount of addons makes it very near possible to recreate actual NASA missions from Mercury to the ISS or launch satellites and probes to other planets that actually communicate LOS with delays and collect science.
Their are three play modes: Sandbox, Science, and Career and you can select difficulty level or define a custom level.
other things aside from building and launching rockets are aircraft and spaceplanes where you can designs things that resemble planes but may or may not fly lol. You can do experiments and get science points which pay for more ship parts, you can set the PR setup in the administration building or goto mission control and select a mission to fly. You even get to recruit Kerbals and select who flys your epic ship to space.
game is not free though and is on the web or on steam, I have the non steam version so I can play it without having steam connected to the internet.

Currently in this game I have a working GPS constellation and my planet is covered in communications via Geo Sync satellites so no matter where a probe is in the solar system it has a link back to mission control IF its not in the shadow of a planet.